Minimizing Alcohol Use In High Efficiency Alcohol Boosted Gasoline Engines

ABSTRACT

A number of systems and methods are disclosed which increase the replenishment interval for anti-knock fluid. This is especially important during activities which require a large amount of anti-knock fluid, such as towing. In some embodiments, the systems and methods are used to reduce anti-knock fluid consumption. For example, changes to engine operation, such as rich operation, spark retarding, upspeeding, and variable valve timing, all serve to reduce the amount of anti-knock fluid required to eliminate knocking. In other embodiments, the composition of the anti-knock fluid is modified, such as by using a higher octane fluid, or through the addition of water to the anti-knock fluid. In other embodiments, the replenishment interval is increased through a larger anti-knock fluid storage capacity. In one embodiment, a three tank system is used where the third tank can be used to store gasoline or anti-knock fluid, depending on the driving conditions.

This application is a continuation of U.S. patent application Ser. No.12/555,911, filed Sep. 9, 2009, which claims priority to U.S.Provisional Patent Application No. 61/096,604, filed Sep. 12, 2008, thedisclosures of which are hereby incorporated by reference in theirentireties.

BACKGROUND

Increasing concerns about global climate change and energy security callfor cost effective new approaches to reduce use of fossil fuels in carsand other vehicles. Recent domestic legislation, as well as the Kyotoprotocol for greenhouse gas reduction, set challenging goals forreduction of CO₂ emissions. For example, the California legislationphases in requirements for reducing CO₂ generation by 30% by 2015. Otherstates may follow California in establishing lower emission goals. Whilenew technologies, such as electric vehicles, are being pursued, costeffective approaches using currently available technology are needed toachieve the widespread use necessary to meet these aggressive goals forreduced fossil fuel consumption. Ethanol biofuel could play an importantrole in meeting these goals by enabling a substantial increase in theefficiency of gasoline engines.

One method of improving traditional gasoline engine efficiency isthrough the use of high compression ratio operation, particularly inconjunction with smaller sized engines. The aggressive turbocharging (orsupercharging) of the engine provides increased boosting of naturallyaspirated cylinder pressure. This pressure boosting allows a stronglyturbocharged engine to match the maximum torque and power capability ofa much larger engine. Thus, the engine may produce increased torque andpower when needed. This downsized engine advantageously has higher fuelefficiency due to its low friction, especially at the loads used intypical urban driving.

Engine efficiency can also be increased by use of higher compressionratio. Compression ratio is defined as the ratio of the total volume ofthe cylinder when the piston is at the top of its stroke, as compared toits volume when the piston is at the bottom of its stroke. Liketurbocharging, this technique serves to further increase the pressure ofthe gasoline/air mixture at the time of combustion.

However, the use of these techniques is limited by the problem of engineknock. Knock is the undesired rapid gasoline energy release due toautoignition of the end gas, and can damage the engine. Knock most oftenoccurs at high values of torque, when the pressure and temperature ofthe gasoline/air mixture exceed certain levels. At these hightemperature and pressure levels, the gasoline/air mixture becomesunstable, and therefore may combust in the absence of a spark.

Octane number represents the resistance of a fuel to autoignition. Thus,high octane gasoline (for example, 93 octane number vs. 87 octane numberfor regular gasoline) may be used to prevent knock and allow operationat higher maximum values of torque and power. Additionally, otherchanges to engine operation, such as modified valve timing may alsohelp. However, these changes alone are insufficient to fully realize thebenefits of turbocharging and higher compression ratio.

The use of higher octane fuels can reduce the problem of knocking. Forexample, ethanol is commonly added to gasoline. Ethanol has a blendingoctane number of roughly 110, and is attractive since it is a renewableenergy source that can be obtained using biomass. Many gasoline mixturescurrently available are about 10% ethanol by volume. However, thisintroduction of ethanol does little to affect the overall octane of themixture. Mixtures containing higher percentages of ethanol, such as E85,suffer from other drawbacks. Specifically, ethanol is more expensivethan gasoline, and is much more limited in its supply. Thus, it isunlikely that ethanol alone will replace gasoline as the fuel forautomobiles and other vehicles. Other fuels, such as methanol, also havea higher blending octane number, such as 130, but suffer from the samedrawbacks listed above.

It is known that the direct injection of an anti-knock fluid havingalcohol content (such as ethanol or methanol) into the cylinder has astabilizing effect on the gasoline/air mixture and reduces thepossibility of knocking. In some embodiments, the anti-knock fluid mayalso include gasoline and/or water. FIG. 5 shows a representative boostsystem.

The boost system 100 includes a spark ignition engine 105, incommunication with a manifold 110. The manifold 110 receives compressedair from turbocharger 120, and gasoline from gasoline tank 130. Thegasoline and air are mixed in the manifold 110, and enter the engine105, such as through port fuel injection. A second tank 140 is used tohold anti-knock fluid, which enters the engine 105 through directinjection. Additionally, the boost system 100 includes a knock sensor150, adapted to monitor the onset of knock. The system also includes aboost system controller 160. The boost system controller receives aninput from the knock sensor 150, and based on this input, controls therelease of anti-knock fluid from the second tank 140 and the release ofgasoline from the gasoline tank 130. In some embodiments, the boostsystem controller 160 utilizes open loop control to determine the amountof gasoline and anti-knock fluid to inject into the engine 105. Inanother embodiment, a closed loop algorithm is used to determine theamount of anti-knock fluid, based on the knock sensor 150, and suchparameters as RPM and torque.

Ethanol has a high fuel octane number (a blending octane number of 110).Moreover, appropriate direct injection of ethanol, or otheralcohol-containing anti-knock fluids, can provide an even largeradditional knock suppression effect due to the substantial air chargecooling resulting from its high heat of vaporization. Calculationsindicate that by increasing the fraction of the fuel provided by ethanolup to 100 percent when needed at high values of torque, an engine couldoperate without knock at more than twice the torque and power levelsthat would otherwise be possible. The level of knock suppression can begreater than that of fuel with an octane rating of 130 octane numbersinjected into the engine intake. The large increase in knock resistanceand allowed inlet manifold pressure can make possible a factor of 2decrease in engine size (e.g. a 4 cylinder engine instead of an 8cylinder engine) along with a significant increase in compression ratio(for example, from 10 to 12). This type of operation could provide anincrease in efficiency of 30% or more. The combination of directinjection and a turbocharger with appropriate low rpm response providethe desired response capability.

Because of the limited supply of ethanol relative to gasoline and itshigher cost, and to minimize the inconvenience to the operator ofrefueling a second fluid, it is desirable to minimize the amount ofethanol, or alcohol-based anti-knock fluid, that is required to meet theknock resistance requirement. By use of an optimized fuel managementsystem, the required ethanol energy consumption over a drive cycle canbe kept to less than 10% of the gasoline energy consumption. This lowratio of ethanol to gasoline consumption is achieved by using the directethanol injection only during high values of torque where knocksuppression is required and by minimizing the ethanol/gasoline ratio ateach point in the drive cycle. During the large fraction of the drivecycle where the torque and power are low, the engine would use onlygasoline introduced into the engine by conventional port fueling. Whenknock suppression is needed at high torque, the fraction of directlyinjected ethanol is increased with increasing torque. In this way, theknock suppression benefit of a given amount of ethanol is optimized.

In one embodiment, an anti-knock fluid, such as an alcohol (such asethanol or methanol) or alcohol blend with water and/or gasoline, iskept in a container separate from the main gasoline tank. As shown inFIG. 5, boost fluid from a small separate fuel tank is directly injectedinto the cylinders (in contrast to conventional port injection ofgasoline into the manifold). The concept uses the direct fuel injectortechnology that is now being employed in production gasoline enginevehicles. The traditional path used by the gasoline is maintained, andis used to aspirate the gasoline prior to its injection into thecylinder. In situations where knocking may occur, such as high torque ortowing, the anti-knock fluid is injected directly into the cylinder. Thehigh heat of vaporization of the boost gas reduces the temperature ofthe gasoline/air mixture, thereby increasing its stability. Insituations where knocking is not common, such as normal highway driving,the anti-knock fluid is not used. Thus, by limiting the use of theanti-knock fluid to only those situations where knocking is prevalent,the amount of anti-knock fluid used can be minimized.

By directly injecting the anti-knock fluid into the cylinder, knockingcan be significantly reduced. This allows boost ratios of 2 to 3 andcompression ratios in the 11 to 14 range. A fuel efficiency increase of20%-30% relative to port fuel injected engines can be achieved usingthese parameters. Alcohol boosting can provide a means to obtain rapidpenetration of high efficiency engine technology in cars and light dutytrucks.

As noted above, the anti-knock fluid is kept in a separate container,and therefore may need to be replenished periodically. In someembodiments, it may be necessary for the operator to perform thisfunction. If the need to replenish the anti-knock fluid is infrequent,such as is the case of windshield wiper fluid, the inconvenience isminimal. However, if the anti-knock fluid needs to be refilled often,this may present an unacceptable solution to consumers.

Therefore, it would be advantageous if a turbocharged spark-ignitedengine could be injected with anti-knock fluids that eliminate theknocking issue, while remaining low cost and readily available. Theengine could operate using gasoline, natural gas or any other fluidappropriate to spark-ignited engines. Furthermore, it would bebeneficial if the use of this anti-knock fluid were minimized so as toreduce the frequency of replenishment.

SUMMARY

A number of systems and methods are disclosed which increase thereplenishment interval for anti-knock fluid. This is especiallyimportant during activities that require a large amount of anti-knockfluid, such as towing. In some embodiments, the systems and methods areused to reduce anti-knock fluid consumption. For example, changes toengine operation, such as rich operation, spark retarding, upspeeding,and variable valve timing, all serve to reduce the amount of anti-knockfluid required to eliminate knocking. In other embodiments, thecomposition of the anti-knock fluid is modified, such as by using ahigher octane fluid, or through the addition of water to the anti-knockfluid. In other embodiments, the replenishment interval is increasedthrough a larger anti-knock fluid storage capacity. In one embodiment, athree tank system is used where the third tank can be used to storegasoline or anti-knock fluid, depending on the driving conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 compares the ethanol usage to the methanol usage needed toeliminate knock in an engine operating at 1000 RPM, and having acompression ratio of 10, as function of the manifold pressure;

FIG. 2 compares the ethanol usage to the methanol usage, as the mixturesare diluted with water, needed to eliminate knock in an engine operatingat 1000 RPM and having a compression ratio of 10;

FIG. 3 compares the ethanol usage needed to eliminate knock as afunction of manifold pressure for homogeneous and stratified charging;

FIG. 4 compares ethanol usage needed to eliminate knock for twodifferent compression ratios and for direct injected gasoline, for anengine operating at 1000 RPM;

FIG. 5 shows a boost system;

FIG. 6 shows the boost system of FIG. 5, as modified to reduceanti-knock fluid consumption; and

FIG. 7 shows a three compartment fuel storage system to increaseanti-knock fluid replenishment time.

DETAILED DESCRIPTION OF THE INVENTION

The fluid (i.e. ethanol) boosted gasoline engine facilitates realizationof the full potential for highly pressure boosted, high compressionratio spark ignited engine operation by greatly alleviating the knockconstraint. This is accomplished by appropriately controlled directinjection of an anti-knock fluid, such as ethanol or ethanol blends,into the cylinder. Direct injection of ethanol acts as an effectivepowerful knock suppressant. The fraction of the fuel provided by theethanol may be varied instantaneously according to the need for knocksuppression. This need may be non-existent at low torque where knocksuppression is not needed and may be as high as 100% when maximum knocksuppression is needed at high torque.

In order to reduce any inconvenience of using a second anti-knock fluid,alcohol boosted operation should preferably be configured such that thedriver has the option of refilling the second tank as infrequently asonce every 5,000 miles, or preferably once every 10,000 miles. Thisreplenishment interval may correspond to the regular servicing at thedealer or a garage. It would also be desirable for the driver to havethe option of refilling one or more containers of alcohol, and to havethis service performed either by the driver or by a service stationattendant at an interval of approximately every 2,000 miles. In order tomeet these goals, the alcohol consumption should be less than 2% of thegasoline consumption over a typical drive cycle.

One particular challenge for alcohol boosted operation is the large rateof alcohol consumption in applications which may require prolonged hightorque operation, such as prolonged towing with a pick up truck.Therefore, it would be beneficial to define a plurality of differentoperating modes. For example, one mode may optimize efficiency at lowtorque, where only the gasoline and small amounts of anti-knock fluidare being used to fuel the engine. A second mode may be to minimize theconsumption of anti-knock fluid in situations involving prolongedperiods of high torque, such as towing. Other modes may also be defined.

A number of methods to reduce the consumption of the anti-knock fluidare described herein to reduce the interval for replenishment ofanti-knock fluid. Methods to reduce anti-knock fluid consumptioninclude:

-   -   Use of methanol in the second tank: the use of methanol in the        secondary tank instead of ethanol can halve the boost fuel        requirements.    -   Use of alcohol/water mixtures: Alcohol/water mixtures can        substantially decrease the consumption of the anti-knock fluid.        Water and alcohol/water mixtures have substantially higher heat        of vaporization than the neat alcohols. Use of an alcohol/water        mixture could provide about a factor of four reduction in the        anti-knock fluid consumption, as compared to E85.    -   Engine operation modification: It is possible to decrease the        alcohol consumption by modifying engine operation, such as by        operating at higher revolutions per minutes (RPMs) than the        engine would normally use (up-speeding) or use of charge        stratification.    -   Aggressive spark retard: Aggressive spark retard can be used to        decrease the consumption of the anti-knock fluid. Although        limited spark retard has a minimal adverse effect on efficiency        while decreasing the alcohol consumption, aggressive spark        retard continues to decrease alcohol consumption but adversely        affects the engine efficiency and increases the exhaust gas        temperature.    -   Use of premium fuel in main tank: A decrease in the consumption        rate of the anti-knock fluid in the secondary tank can be        achieved if there is premium fuel in the primary tank. As        premium fuel is broadly available, its use to achieve alcohol        consumption reduction has minimal impact on the consumer. Use of        premium fuel would be needed only for extended towing        applications.    -   Variable Valve Timing (VVT): Variable valve timing can be used        to decrease the compression ratio at conditions of high load,        decreasing the tendency of the engine to knock and the amount of        anti-knock fluid required.    -   Direct injection of gasoline and anti-knock fluid: It is        possible to directly inject both gasoline and the anti-knock        fluid, thereby decreasing the amount of anti-knock fluid        required to prevent knock.

These techniques may be employed upon detection of a change in vehicularoperating condition. “Vehicular operating condition” is defined as theprolonged operating condition of an engine over time. For example, ineveryday driving, the engine may experience periods of high torque (suchas acceleration) and low torque (high speed cruising). However, whenviewed over an extended period, the average torque is relatively low,and therefore the consumption of anti-knock fluid is also relativelylow. In contrast, when a vehicle is under high stress, such as whentowing, the engine may experience high torque continuously. Thus, whenviewed over an extended period of time, the average torque is muchhigher. This much higher average torque leads to increased consumptionof anti-knock fluid. It is anticipated that brief changes in torque,such as acceleration or climbing, are not considered changes in thevehicular operating condition. Rather, it is the prolonged change inengine torque that precipitates a change in vehicular operatingcondition. While towing is the most common cause of prolonged hightorque, other conditions may result in prolonged high torque and arewithin the scope of the disclosure.

Alternatively, the capacity of the second tank can be increased, therebyincreasing the replenishment interval. In some embodiments, the largeramounts of anti-knock fluid may be at the expense of gasoline.

One method to reduce anti-knock fluid consumption is to use ananti-knock fluid having a higher blending octane number and whichprovides greater vaporization cooling than ethanol. Methanol is anattractive cooling fluid due to its greater knock suppression effect,its ready availability in containers (such as in methanol/water mixturesfor windshield cleaner or as “dry gas” for prevention of fuel linefreeze up), and its potential production from biomass and coal as wellas from natural gas.

Methanol has a higher blending octane than ethanol (roughly 130 ascompared to 110), thereby reducing its consumption as compared toethanol.

Calculations of the ethanol consumption have been previously carried outin order to evaluate the requirements of the second fuel tank thatcontains the alcohol, and to develop refueling approaches. Ethanolrequirements were previously calculated using a simple model, describedin [Bromberg, Cohn, Heywood, Laboratory for Energy and the EnvironmentReport LFEE-2006-01 (2006)], the disclosure of which is hereinincorporated by reference in its entirety, which determines the amountof alcohol required through the engine map, and then uses a vehiclemodel to determine the average utilization over conventional drivingcycles (HWFET, UDDS, us06). The model was then used to determine theethanol consumption for different levels of turbocharging andcompression ratios, using ethanol as the anti-knock fluid.

This model has been used to calculate the ratio of alcohol to gasolineconsumption for a representative case for prolonged high torqueoperation. FIG. 1, which was generated using this model, shows thescenario of low RPM methanol and ethanol requirements, as a function ofpressure in the inlet manifold. The conditions of FIG. 1 are for anengine speed of 1000 rpm and an engine with a compression ratio of 10.Line 10 of FIG. 1 shows the ratio of consumption of ethanol to gasoline,by volume. Line 20 shows the ratio of consumption of methanol togasoline, by volume. It is assumed that the ethanol is E100, and themethanol is M100; while the gasoline is modeled as 87 PRF. Smalldilutions of the alcohols by gasoline will not change substantially thecomparison. The calculation is performed for the case of homogeneousinjection of the alcohol and port fuel injection of the gasoline.

The ratio of the refueling interval of the alcohol tank to that of thegasoline tank is inversely proportional to the ratio of alcohol rate ofconsumption to the rate of consumption of gasoline. It should bestressed that under normal driving conditions the alcohol consumptionneeds to be calculated over a driving cycle. FIG. 1 shows the curvesonly for a given set of speed and torque, for conditions where thepropensity to knock is the greatest and the alcohol requirement isconsequently the highest.

For a manifold pressure of around two bar, the methanol consumption isapproximately equal to that of gasoline and about half that of ethanol.Thus, using methanol should approximately double the replenishmentinterval of the anti-knock fluid tank. The advantage of methanol furtherincreases at higher values of inlet manifold pressures. For example, ata manifold pressure of three bar, ethanol consumption is seven timesthan of gasoline, and 3.5 times that of methanol. Line 20 also showsthat methanol is far less sensitive to the manifold pressure thanethanol.

The alcohol fraction under the conditions of FIG. 1 is high even withthe use of methanol. This is because of the assumptions used in thecalculations, for example, very low engine speeds and high torque, whichare conditions that exacerbate knock. FIG. 1 shows the trends that maybe expected to exist at more realistic towing conditions, at higherspeeds.

In another embodiment, water can be added to the anti-knock fluid (suchas ethanol or methanol) to further reduce the anti-knock fluidrequirement.

Alcohol/water mixtures have substantially higher heat of vaporizationthan pure alcohols. Although water can have an even larger effect on thetemperature, the large amounts required result in difficulty in sparkingand initial flame propagation, increasing misfire.

FIG. 2 shows the required fraction of the anti-knock fluid at low enginespeed and high boost pressure (2 bar absolute in the manifold) toprevent knock, using the model described in reference to FIG. 1. FIG. 2shows the results for an ethanol/water mixture on line 30 and amethanol/water mixture on line 40. These lines represent the volumeratio of the anti-knock fluid to gasoline, as a function of the alcoholfraction. The test parameters included an engine with a compressionratio of 10, operating at 1000 RPM with 2 bar manifold pressure. Thepoint at the far left shows the volume ratio of pure water to gasoline,while the far right shows these ratios for pure alcohols. The graphrepresents various levels of dilution of the alcohol/water mixture. Inother words, FIG. 2 shows that, for a mixture which is 70% anti-knockfluid and 30% water, the ratio of methanol to gasoline is approximately0.75, while the ratio of ethanol to gasoline is about 1.

The amount of pure methanol needed at all levels of dilution is lessthan the amount of necessary pure ethanol. Furthermore, the amount ofethanol/water or methanol/water mixtures decreases monotonically withincreased water content (or more dilute alcohol mixtures). In otherwords, the addition of water to the mixture has two effects. First, thewater replaces the anti-knock fluid, thereby reducing the amount ofanti-knock fluid in the mixture. Second, the presence of water decreasesthe amount of the mixture needed to eliminate knock.

For example, according to line 40 on FIG. 2, about 1.5 gallons of puremethanol per gallon of gasoline are needed in this drive scenario.However, only about 0.5 gallons of anti-knock fluid are needed pergallon of gasoline at a dilution of 0.2. Therefore, it is possible todecrease the amount required by about a factor of 4.

Another approach for reducing the alcohol use is to operate the engineat higher revolutions per minutes, also known as upspeeding. Operatingat higher RPMs reduces the engine's tendency to knock, and thus thealcohol requirement, due to a decrease in combustion time. These shortercombustion times reduce the time in which the autoignition of theend-gas may occur. Shorter combustion times are due to increasedturbulence in the engine.

A simple model has been used to determine the effect of increased enginespeed and decreased torque (for constant mechanical power from theengine). The results are shown in Table 1. The table indicates a verylarge decrease in ethanol consumption by operating at higher speed andlower torque, but at the same power conditions. It is assumed thatcombustion time also decreases with increasing engine speed. In Table 1,it is assumed that the combustion time is constant in crank-angledegrees at the higher speed conditions, which may be an optimisticcondition. However, the gain is very large, and the speed of combustionis increasing, although not necessarily at the same rate as the enginespeed.

TABLE 1 Manifold Engine speed pressure Ethanol consumptior (rpm) (bar)(ethanol/gasoline) 3000 2 0.67 4000 1.5 0.08

In addition to the above scenario, a more detailed calculation has beencarried out for a large heavy duty engine, described in [Blumberg,Bromberg, Kang, Tai, SAE 2008-01-2447], the contents of which is hereinincorporated by reference in its entirety. That calculation evaluated an11 liter heavy-duty engine at several operating points. The compressionratio used is 14. The results are shown in Table 2, for constant power.Brake Mean Effective Pressure (BMEP) is a well-known measure of anengine's capacity to do work, and is independent of its displacement.Thus, by normalizing the BMEP of these two operating conditions to anarbitrary value, the results can be properly computed. Maximum torque isachieved at 1500 rpm.

TABLE 2 Ethanol Engine Normalized consumption (rpm) BMEP (E85/gasoline)BTE 1500 1.3 1.44 39.6% 1800 1.08 0.64 36.9%

Therefore, even in the case of the large engine model, there is asubstantial decrease in the consumption of the secondary fuel by movingto higher engine speed and lower BMEP, but at approximately constantpower. In the case of Table 2, the decrease in ethanol consumption isabout 60% for a 20% increase in engine speed. In comparison, Table 1showed a decrease in ethanol consumption of about 90% for a 30% increasein engine speed. The engine efficiency (or Brake Thermal Efficiency—BTE)is 39.6% for the lower engine speed and about 36.9% for the higherengine speed.

In both engines, there is a substantial decrease in ethanol consumptionin conditions of medium to high power by going to higher engine speedoperation.

Operation at higher engine speeds can be achieved by adjusting thepowertrain gear ratios of the vehicle in order to provide theappropriate gear for towing.

For example, the operator/driver, or the boosting system controller maydetermine that a high torque situation exists. In one embodiment, thedriver determines this, and supplies this information to the boostingsystem controller. This can be done using an actuator, such as a buttonor lever on the dashboard or gear box. This may be similar to the methodused to indicate the need for four wheel drive on some automobiles, orover-drive. In other embodiments, the boost system controller determinesthat a high torque situation exists. This can be done by monitoring theconsumption rate of anti-knock fluid being injected into the cylinders,or by monitoring the torque being generated by the engine. High torqueduring the vehicular operating condition can be defined an average rateof consumption of the second fluid during a given time period. Theaveraging time can be as short at 10 minutes and as long as 1 hour. Thedetermination of high torque can be made based on the consumption ratebeing greater than a predetermined value. In another embodiment, hightorque may be determined based on the ratio of the consumption rate ofthe anti-knock fluid to the consumption rate of the gasoline. If thisratio exceeds a predetermined value for a time period greater than apredetermined limit, a determination of high torque is made.Alternatively, the consumption rates of anti-knock fluid and gasolinemay be continuously averaged over a predetermined time period. If theratio of the average consumption rate of anti-knock fluid to the averageconsumption rate gasoline exceeds a predetermined threshold, adetermination of high torque is made.

FIG. 6 shows a boost system 200 used in one embodiment. Those componentswith the same function as used previously in FIG. 5 are given likenumbers. In this embodiment, the boost system controller 260 receivesvarious inputs, such as a signal from the boost sensor 150 and from theoperator 270. Based on these inputs, the boost system controller 260determines that the amount of anti-knock fluid being used is above apredetermined threshold, which indicates a high torque situation. Once ahigh torque situation has been determined, the boosting systemcontroller 260 may signal the automatic transmission system 230. Thissignal would inform the automatic transmission system 230 to shift to alower gear, so as to increase the engine speed while decreasing thetorque. Methods to affect such a change are well known, and arecurrently used in response to various situations, such as accelerationand uphill driving.

The boosting system allows higher torque operation at lower RPMs thanconventional engines. Therefore, it may also be desirable to introduce adifferent gear ratio for the top gear. This gear ratio would allow lowerengine speed at a given vehicle speed, thereby minimizing gasolineconsumption for highway driving, but increasing use of the secondaryfluid. Such a gear ratio may be such that the ratio of engine speed tovehicle speed is about 25-30% lower in top gear than is now customary.This gearing would be used in normal cruising (lightly loaded vehicle),and is an effective tool for increasing the vehicle fuel economy.

Another method of reducing the amount of anti-knock fluid needed in hightorque situations is the use of charge stratification of the secondaryfluid as a means of controlling knock. Autoignition of the gas typicallyoccurs away from the spark, typically at the far end, or along thewalls, of the cylinder. Therefore, it is most essential that theanti-knock fluid be added to these regions of the cylinder towards theend of the combustion phase, while it is less important that it reachthe center or lower portion of the cylinder. One method to stratify thecharge is to introduce the anti-knock fluid late in the engine cycle, sothat it does not dissipate fully before the spark ignition. Althoughlate injection is a way to produce charge stratification, it isadvantageous to inject early (and to have evaporation take place afterinlet-valve-closing) to make the best use of the high heat ofvaporization of the anti-knock fluid in order to maximize the decreaseof end-gas temperature.

One way to maintain charge stratification may be to inject the alcoholin the periphery of the cylinder, with substantial air swirl but littleor preferably no tumble. In this case, thermal stratification can beused to maintain the charge stratification. The heavier, cooler air/fuelmixture will stay in the periphery of the cylinder by centrifugal forcesdue to the strong swirl.

Charge stratification decreases the amount of ethanol required (as afraction of the gasoline required) by about a factor of 2 at manifoldpressures of about 2. FIG. 3 demonstrates this trend. Line 50 shows theamount of ethanol needed to eliminate knock in a homogenous cylinder,while line 60 shows the amount needed if the ethanol is stratified. Itshould be noted that the consumption of ethanol has been substantiallyreduced, and the savings increase at higher manifold pressures.Furthermore, the large cylinders in heavy duty vehicle engines allow forbetter stratification of the ethanol.

Another method that can be used to substantially decrease the anti-knockfluid consumption is spark retard, which can be accomplished with verysmall effect on fuel efficiency. Tests have shown that a 7 degree retardin Crank Angle (CA) in spark time from MBT (Maximum Brake Torque)decreases the engine efficiency (as measured by BTE) by slightly overone percent. However, it also results in a significant reduction in theamount of ethanol used. Table 3 shows the results, where the engineconditions correspond to the B100 point in the ESC (European StationaryCycle) mode test, corresponding to intermediate speed (1547 rpm) andmaximum torque at that speed (in this case, 32.9 bar BMEP), as describedin [Blumberg, Bromberg, Khan and Tai, SAE 2008-01-2447], the disclosureof which is herein incorporated by reference in its entirety.

TABLE 3 CA10-90 CA50 E85/gasoline (CA deg) (CA deg) BTE ratio (by vol)MBT 25 8 42.5%   11.7 Moderate spark retard 25 14.5 41% 2.54 Aggressiveretard 25 24 38% 1.1

Three scenarios are presented in Table 3, and include the MBT case,moderate spark retard and aggressive spark retard. The combustion speed(noted as CA10-90, the duration, in crank-angle (CA) degrees, betweencombustion of 10% and 90% of the fuel) has been held constant. CA50indicates the timing (in crank angle after Top Dead Center, where 50% ofthe fuel has been consumed. However, the timing of peak pressure ischanged because of the spark retard. As mentioned before, moderate sparkretard does not affect the efficiency significantly, changing the BTEfrom 42.5% to 41%. Further increases in spark retard drop the BTE toabout 38%. However, the moderate spark retard dramatically decreases theethanol/gasoline ratio (by volume), from about 12-to-1 at MBT to about2.5-to-1. The moderate spark retard results in boost consumption ratiobeing reduced by a factor of 4.5. Further retardation of the spark to 24degrees further decreases the ethanol consumption by another factor of 2and the absolute ethanol consumption by about a factor of ⅓.

Aggressive spark retard can be optionally used under conditions ofprolonged high torque operation. The initiation of spark retard can beimplemented by the operator 270 or the boost system controller 260 asdescribed above. FIG. 6 shows the boost system controller 260 supplyingan input to engine 105. This input is used to instruct the engine 105 tointroduce spark retard, and optionally to specify the amount of retardto be used (e.g. moderate or aggressive).

Another method of reduce the consumption of anti-knock fluid is byincreasing the ratio of gasoline to air in the vehicle (also known asrich operation). Rich operation can be used to extend the knock freeregion in conventional and directly injected engines.

Table 4 shows the effect of increased fuel-to-air ratio (expressed asequivalence ratio, Φ, which is the ratio between the fuel-to-air ratiodivided by the fuel-to-air ratio at stoichiometric combustion). In otherwords, an equivalence ratio, Φ, indicates normal or typical operation,while a value of 1.1 indicates a 10% increase in the amount of fuel inthe fuel-air mixture. Table 4 is calculated at the B100 point,corresponding to about 1547 rpm and 32.9 bar BMEP.

TABLE 4 ethanol/gasoline Φ ratio (by vol) BTE 1 2.60 41.0% 1.05 2.4939.4% 1.1 2.20 37.5% 1.15 1.95 35.7%

Although not as strong reduction as some of the other means described,there is still a 20% reduction in ethanol consumption ratio by going toenrichments of Φ˜1.1, with a decrease in fuel efficiency, as not all theenergy of the fuel is consumed. If this mode of operation is usedsporadically, the average fuel consumption of the vehicle should not besignificantly impacted, but it allows the operator the flexibility ofbeing able to reduce the consumption of the secondary tank fluid at theexpense of increased consumption of the main tank for short periods oftime. Note that increases in Φ adversely affect engine efficiency (asmeasured by BTE). As with other parameters, changes in the compositionof the gasoline/air mixture can be determined by the driver 270 or theboost system controller 260. FIG. 6 shows an input to the manifold 110from the boost system controller 260, which can be used to alter thegasoline/air mixture, as necessary.

The use of premium fuel can also be used to reduce the rate ofutilization of the alcohol fuel. The effect of using premium incomparison to regular octane gasoline is summarized in Table 5.

TABLE 5 Gasoline ethanol/gasoline octane ratio (by vol) 87 1.56 93 1.44

The results of Table 5 were calculated for an engine speed of 1000 rpmand 2 bar inlet manifold pressure. These are the worst conditions forknock in a spark ignited engine. The gasoline was modeled using primaryreference fuels (iso-octane and n-heptane),

Two values of gasoline octane were investigated, 87 (corresponding toregular gasoline) and 93 (corresponding to premium). The ratio ofethanol to gasoline consumption was slightly decreased, by about 10%, bythe use of higher octane gasoline.

Another method to conserve anti-knock fluid is by variation of thecompression ratio. The knock limit results in severe limitation to theallowable compression ratio in spark ignited engines. The use ofdirectly injected anti-knock fluids can prevent knock, thereby allowinghigher compression ratios. While higher compression ratios result inincreased fuel economy, they may also require larger amounts ofanti-knock fluid to operate properly.

In situations where anti-knock fluid should be conserved, it is possibleto decrease the maximum compression ratio, either with variablecompression ratio or with variable valve timing (VVT).

Variable compression ratio may be achieved through the use of cylinderswith an adjustable volume. In some embodiments, the cylinder head can beadjusted, allowing the volume within the cylinder to change.

Variable valve timing is a means to control the point in time (relativeto the piston stroke) at which air and exhaust enter and exit thecylinder. By modifying this timing, it is possible to effectively changethe compression ratio.

The ethanol requirement as a function of the manifold pressure is shownin FIG. 4 for two different values of compression ratio. Line 70represents the ethanol fraction required to eliminate knocking at acompression ratio of 12. Line 80 represents the ethanol fractionrequired to eliminate knocking at a compression ratio of 10. From theselines, it can be seen that the amount of ethanol required is decreasedby about 15% when the compression ratio is decreased from 12 to 10.Further decreases in compression rate would yield further ethanolsavings.

To effect this change, the driver 270 or boost system controller 260 maydetermine that high torque or towing conditions exist, and signal thisstatus to the engine, as described above. FIG. 6 shows one or moreinputs to the engine 105 from the boost system controller 260. This setof inputs can be used for spark retard timing (as described above),variable valve timing, variable compression ratio modification, or anycombination thereof.

Another method to extend the replenishment interval for anti-knock fluidis through direct injection of gasoline. Although the cooling effect ofdirectly injected gasoline is substantially lower than that of thealcohols, the use of directly injected gasoline also results in areduction in the consumption of anti-knock fluid. The use of combineddirect injection of ethanol and gasoline is shown in line 90 in FIG. 4,for an engine with a compression ratio of 10. Separate injectors forgasoline and anti-knock fluid may be relatively easy to implement in thelarge cylinders used for heavy duty vehicle engines. In someembodiments, gasoline can be direct injected into the cylinder.

While the above methods and systems are concerned with minimizinganti-knock fluid consumption, another approach is to increase theanti-knock fluid capacity in the vehicle. In one embodiment, a towingpackage option is provided that would include another tank foranti-knock fluid. This additional tank would be in addition to theanti-knock fluid storage tank that is part of the standard package. Thissecond anti-knock fluid tank could increase the ethanol storage capacityby a factor of two and thus the alcohol refill interval by a factor oftwo.

In another embodiment, shown in FIG. 7, a fuel tank system 300 withthree compartments where one of the compartments can be filled withdifferent fuels can be used. The first tank 310 is filled with gasoline.The second tank 320 is used for anti-knock fluid for the octaneboosting. The third compartment 330, which can be connected with eitherthe first or second tanks, is filled with either gasoline or ethanol. Insome embodiments, the third compartment 330 is connected to the gasolinetank 310 such as with a valve 340. Similarly, third compartment 330 maybe connected to anti-knock fluid tank 320 via a second valve 350. Inordinary driving conditions where anti-knock fluid consumption isrelatively low, the third compartment 330 is connected to the first tank310 and is filled with the same filling system as the first tank 310.Under conditions of prolonged towing, the third compartment 330 isconnected to the second tank 320 and filled with the same filling systemas the second tank 320. The selection of which tank the thirdcompartment should be connected to may be made by the boost systemcontroller 260. In one embodiment, outputs from the boost systemcontroller 260 are used to provide input to valves 340, 350. Thisdetermination may be based on prior driving habits. In anotherembodiment, the selection of which tank the third compartment should beconnected to is made by the driver, who is aware of whether the vehiclewill experience a prolonged period of high torque usage.

When the third compartment 330 is used to store gasoline, gasoline isfirst consumed from the third compartment 330 before it is consumed fromthe first tank 310. In this way, the third compartment 330 becomesavailable for filling with anti-knock fluid as quickly as possible. Evenif the third compartment 330 contains some amount of gasoline, refillingit with anti-knock fluid can extend the refill interval for the secondtank 320.

If the second tank 320 and third compartment 330 each have the samevolume, the anti-knock fluid refill time can be increased by a factor oftwo by using both tanks to store alcohol, at the expense of the need torefill the gasoline tank 310 more often.

Each of the methods and systems described above can be used to increasethe replenishment interval for anti-knock fluid. Each method can be usedalone, or in conjunction with one or more other methods. Thus, thesavings described in for each method may be multiplied to achieve evenhigher overall savings.

While ethanol has been described in some portions of the specification,it is to be understood that ethanol and methanol blends are alsocovered. The blending agents could be water and/or gasoline, or otherfluids for lubrication, freeze-prevention or improved blending.Furthermore, while the specification describes the use of gasoline inspark ignited vehicles, it is to be understood that natural gas andother fuels that operate with spark ignited engines can also be embodiedwith the present invention.

What is claimed is:
 1. A spark ignition engine system, where a firstfluid is introduced into said engine from a first source and a secondfluid is introduced into said engine from a second source and whereinthe ratio of said second fluid to said first fluid varies with enginetorque so as to prevent knock, comprising: a controller configured tomodify the operation of said engine so as to reduce an amount of saidsecond fluid that would otherwise be used to prevent knock, based on achange in vehicular operating condition, wherein said change comprisesprolonged use at high torque.
 2. The spark ignition engine system ofclaim 1, wherein said change in vehicular operating condition is due toprolonged towing.
 3. The spark ignition engine system of claim 1,wherein said modification comprises an increase in spark retard timing.4. The spark ignition engine system of claim 1, wherein saidmodification comprises use of variable valve timing.
 5. The sparkignition engine system of claim 1, wherein said first fluid is mixedwith air prior to being introduced into said engine and saidmodification comprises a change in the ratio of said first fluid to air.6. The spark ignition engine system of claim 1, wherein saidmodification comprises an increase in RPM.
 7. The spark ignition enginesystem of claim 1, further comprising a transmission, wherein saidcontroller instructs said transmission to upspeed.
 8. The spark ignitionengine system of claim 1, wherein said modification comprises chargestratification, wherein a non-uniform distribution of said second fluidis directly injected into said engine.
 9. The spark ignition enginesystem of claim 1, wherein an operator provides input to said controllerregarding said change to said vehicular operating condition therebycausing said controller to modify said operation of said engine.
 10. Thespark ignition engine system of claim 1, further comprising a knocksensor, wherein said knock sensor provides input to said controller tomodify said operation of said engine.
 11. The spark ignition enginesystem of claim 1, wherein said change in vehicular operating conditionis determined based on the consumption rate of said second fluid. 12.The spark ignition engine system of claim 11, wherein said consumptionrate is averaged over a predetermined time period.
 13. The sparkignition engine system of claim 1, wherein said change in vehicularoperating condition is determined based on the ratio of the consumptionrate of said second fluid to the consumption rate of said first fluid.14. The spark ignition engine system of claim 1, wherein saidmodification to said engine operation lowers the efficiency of saidengine with respect to said first fluid.
 15. The spark ignition enginesystem of claim 1, where said second fluid comprises alcohol.
 16. Thespark ignition engine system of claim 1, where said second fluidcomprises water.
 17. The spark ignition engine system of claim 1, wheresaid first fluid comprises gasoline.
 18. The spark ignition enginesystem of claim 1, where said first fluid comprises natural gas.
 19. Thespark ignition engine system of claim 1, wherein said modificationcomprises increased spark retard and upspeeding.
 20. The spark ignitionengine system of claim 1, wherein said second fluid comprises alcoholand said modification comprises direct injection of water with saidsecond fluid.
 21. The spark ignition engine system of claim 1, whereinsaid first fluid comprises premium gasoline.